Induction heating apparatuses have been employed for heating a variety of materials without direct contact therewith. For instance, heat treating of metals and melting of materials may be accomplished by induction heating. Further examples of induction heating applications include, without limitation, annealing, bonding, brazing, forging, stress relief, and tempering. Additionally, powder metallurgy applications may relate to heating of a mold or other member which, in turn, heats a powder metallurgy composition to be melted. Metal or other casting applications may also utilize induction heating. Accordingly, as known in the art, induction heating may be useful in various industries and applications.
For instance, one particular application for induction heating relates to treatment and storage of such hazardous materials and is known as “vitrification.” Hazardous materials may be vitrified when they are combined with glass forming materials and heated to relatively high temperatures. During vitrification, some of the hazardous constituents, such as hazardous organic compounds, may be destroyed by the high temperatures, or may be recovered as fuels. Other hazardous constituents, which are able to withstand the high temperatures, may form a molten state, which then cools to form a stable vitrified glass. The vitrified glass may demonstrate relatively high stability against chemical and environmental attack as well as a relatively high resistance to leaching of the hazardous components contained therein.
One type of induction heating apparatus that has proven to be effective to vitrify waste materials is a cold-crucible-induction melter (CCIM). A cold-crucible-induction melter may typically comprise a water-cooled crucible disposed within an induction coil, or other inductor, usually formed along a spiral path surrounding therearound. Generally, an induction coil carries varying electric currents that generate associated varying electromagnetic fields for inducing eddy currents within electrically conductive materials encountered thereby. The varying electromagnetic fields generated by the current within an inductor may be described as the “flux” thereof.
Waste may be induction heated directly if it is sufficiently electrically conductive and thereby vitrified. However, the waste and glass forming materials used in vitrification systems may be relatively non-electrically conductive at room temperatures. Therefore, an electrically conductive material may be used to initially indirectly heat at least a portion of the waste to a molten state, at which point the waste may become more electrically conductive so that when varying current is conducted through the induction coil, conductive molten waste may be induction heated by way of eddy currents generated therein. Of course, non-electrically-conductive waste materials nearby the electrically conductive molten waste, due to the heat generated therein, may be indirectly heated and thus, melted.
As a further advantage of cold-crucible-induction melter vitrification systems, molten glass within the water-cooled crucible may form a solid layer (skull layer), which inhibits or prevents direct contact of the high temperature molten glass with the interior surface of the crucible. Furthermore, because the crucible itself is cooled with water, in combination with the insulative properties of the skull layer, high-temperature melting may be achieved without being substantially limited by the heat-resistance or melting point of the crucible.
FIG. 1 shows a perspective view of a conventional induction melter 10. Generally, cold-crucible-induction melter 10 includes head assembly 20 affixed to disengagement spool 40 by way of mating lower flange 21 and upper flange 39 of head assembly 20 and disengagement spool 40, respectively. Disengagement spool 40 is affixed to furnace body 30 by way of lower flange 37, which is affixed to the upper flange 31 of the furnace body 30. Head assembly 20 includes off-gas port 12 for removing gasses from the cold-crucible-induction melter 10 during operation, feed port 14 for adding material to the cold-crucible-induction melter 10, and view port 15 for observing the conditions within the cold-crucible-induction melter 10. Furnace body 30 may include cooling tubes 22 disposed therearound, which may be supplied with a cooling medium, such as water, by way of inlet 23 and outlet 25 for cooling the crucible (not shown) and may also include a bottom drain assembly (not shown) for discharging vitrified waste material from the crucible during operation of the cold-crucible-induction melter 10.
FIG. 2A shows a side cross-sectional view of the cold-crucible-induction melter 10 shown in FIG. 1. More particularly, an induction heating system 90 comprising an induction coil 26, a power source 100, and electrical conductors 110 extending therebetween may be configured for delivering heat to the interior of crucible 56. In further detail, induction heating system 90 may include an induction coil 26 disposed generally about the furnace body 30 of the cold-crucible-induction melter 10 as known in the art (cooling tubes 22 have been omitted from FIGS. 2A–2D for clarity). Both electrical conductors 110 and induction coil 26 may be water-cooled, as known in the art. Power source 100 may comprise a variable-frequency power supply, such as a generator-type or a solid state power supply, which is configured for energizing the induction coil 26 with a selectable, alternating electrical waveform having a magnitude and a frequency wherein at least one of the magnitude and frequency is variable. As known in the art, the power source 100 may be operably coupled to or integrally inclusive of a capacitor “bank” or one or more variable capacitors and a transformer that are configured (separately or in combination) for “tuning” (automatically or manually) the resonant frequency of the induction heating circuit with respect to the load (i.e., the material to be heated).
FIG. 2B shows a side cross-sectional view of the cold-crucible-induction melter 10 shown in FIG. 1 including granular material 55, which may be disposed within crucible 56. For instance, granular material 55 may comprise hazardous materials and glass forming materials, without limitation. Also, susceptor 120 may be positioned in contact with the granular material 55 and may be configured for heating, in response to energizing induction coil 26, to a temperature sufficient to melt at least a portion of the granular material 55 proximate thereto. For instance, susceptor 120 may comprise graphite and may be shaped as a ring or as otherwise desired. The presence of a susceptor 120 may be necessary to initially melt at least a portion of the granular material 55, because the granular material 55 may not be electrically conductive when solid. Of course, conversely, if granular material 55 is electrically conductive in a non-molten state, susceptor 120 may be omitted as being unnecessary.
During initial operation of the induction heating system 90 of the cold-crucible-induction melter 10 as shown in FIG. 2B, assuming granular material 55 is not electrically conductive, induction coil 26 carrying an alternating current induces eddy currents within susceptor 120, thus heating susceptor 120. As susceptor 120 increases in temperature, granular material 55 proximate to susceptor 120 may be heated and may form a region of molten material 50 adjacent susceptor 120, as shown in FIG. 2C. Inductive heating by energizing induction coil 26 with an alternating current may then proceed by way of induced electrical currents within the molten material 50, assuming such molten material 50 becomes electrically conductive, in combination with heating of susceptor 120 by way of induced electrical currents therein until substantially the interior of crucible 56 comprises molten material 50, surrounded by skull layer 52, as explained further hereinbelow and shown in FIG. 2D.
Referring to FIG. 2D, granular material 55 may be introduced within cold-crucible-induction melter 10 through feed port 14 and ultimately melted to form molten material 50, which may substantially fill crucible 56. Susceptor 120 (FIGS. 2B and 2C) may be sacrificial, and may substantially oxidize (burn off) or may break into several pieces within molten material 50. As noted previously, crucible 56 may be surrounded by cooling tubes 22 (FIG. 1) for flowing water or gas through in order to cool the crucible 56 during operation, because the temperatures that may be required to vitrify waste materials may exceed the melting point of the crucible 56. The desired steady-state operational temperature for vitrifying waste material may be about 1200° Celsius. Cooling the crucible 56 during heating of the waste may form a skull layer 52 comprising solidified material (previously molten material 50) disposed along the inner surface of the side wall of the crucible 56. The skull layer 52 may be from a few millimeters to several inches in thickness, and may insulate the molten material 50 within the crucible 56 and also inhibit the molten material 50 from directly contacting and damaging the inner surface of the crucible 56. Skull layer 52 may span a relatively extreme temperature gradient between the cooling water temperature within cooling tubes 22, which may be less than about 100° Celsius, and the molten material 50 temperature, which may be greater than about 1000° Celsius. Of course, the relative thickness of the skull layer 52 may vary depending on the thermal environment of the crucible 56.
Also, cold cap 54, comprising granular material 55 and, possibly, condensed off-gas material, may preferably exist upon the upper surface of molten material 50 under preferred conditions. Cold cap 54 may reduce volatization of molten material 50 and may also insulate molten material 50. Impact zone 59 indicates a region of cold cap 54 that granular material 55, shown as entering the cold-crucible-induction melter 10 through feedport 14, may fall upon and accumulate. Dust, volatized material, and evolved gases 57 may exit or move upwardly away from the impact zone 59 of cold cap 54 into the plenum volume 200. Ultimately, dust, volatized material, and evolved gases 57 may subsequently condense, deposit, or settle onto cold cap 54, adhere to the inner wall of disengagement spool 40 or head assembly 20, respectively, or exit the plenum volume 200 through offgas port 12.
Induction coils 26 surrounding crucible 56 may be energized with relatively large alternating currents to induce currents within the waste material to be heated. Typically, induction coils 26 may be fabricated from a highly electrically conductive material, such as copper, and are cooled by water or another fluid flowing therein. As known in the art, waste materials, such as radioactive waste or other waste may be combined with glass forming constituents, heated, and thereby vitrified.
Conventional induction heating systems may be configured for heating in response to a temperature set-point, which may be time-varying. More particularly, conventional induction heating systems may be configured for varying the output power of the power source in relation to an error signal equal to the difference between a desired set-point in relation to a measured temperature of the material to be heated that is measured or indicated by way of thermocouple or optical pyrometer. For example, in one configuration, a desired set-point may be communicated electrically to a proportional, integral, and derivative (“PID”) type control algorithm, including user-settable or auto-setting constants, and the output of the induction heating system may be determined therewith, as known in the art.
As may be appreciated by the above discussion of the operation and configuration of a cold-crucible-induction melter 10, it may be difficult to measure or ascertain the temperature of the molten material 50 therein. Particularly, one conventional approach may include insertion of at least one thermocouple into molten material 50. However, the power source 100 of induction heating system 90 may induce heat within a thermocouple and, therefore, may potentially damage a thermocouple. Alternatively, in another conventional approach for measuring the temperature of the molten material 50, an optical pyrometer may be employed for indicating a temperature of molten material 50. An optical pyrometer, as known in the art, may indicate the temperature of a surface of a material by measuring the energy radiating from a material (for one or more wavelengths) and relating the measured energy, in consideration of the spectral emissivity of the material, to the temperature of the material. However, as best seen in FIG. 2B, a clear viewing path of molten material 50 for operation of an optical pyrometer may be relatively difficult to establish, use, or reliably maintain, because skull layer 52, cooling tubes 22, induction coil 26, cold cap 54, granular material 55, as well as dust, volatized material, and evolved gases 57 may substantially interfere with radiation from molten material 50. Thus, there may be substantial difficulties in obtaining reliable measured temperature information relating to the molten material 50, which may complicate operation of the cold-crucible-induction melter 10.
In the absence of reliable direct temperature measurements of molten material 50, conventional cold-crucible-induction melters may be controlled manually. For example, conventional cold-crucible-induction melters may be controlled by “feel” or by secondary indications such as the “frequency pulling” in relation to the applied frequency of an induction power source 100. Accordingly, it may be desired to control the output of the power source 100 of cold-crucible-induction melter 10 in relation to the temperature of the molten material 50, automatically or otherwise. Thus, there exists a need for an improved apparatus and method for indicating, controlling, or both indicating and controlling or regulating the temperature distribution within a cold-crucible-induction melter.
In view of the foregoing problems and shortcomings with conventional induction heating apparatus and methods of operation thereof, it would be advantageous to provide improved induction heating apparatus and methods of operation thereof.